The homeostasis of naive, effector, and memory T lymphocytes is regulated by cytokines, MHC/peptide ligands, and apoptotic pathways. Proteins belonging to the Bcl-2 family are the major players of the intrinsic apoptotic pathway. Recent studies on the role of the Bcl-2 family in T cell apoptosis suggest that members of this family are the principal regulators of the survival/death pathways that decide the fate of T cells. However, most of the studies have focused on pro-apoptotic BH3-only and multi-domain members, leaving the roles of the anti-apoptotic members (Bcl-2, Bcl-xL, Mcl-1, and A1) in regulating the survival of naive, effector, and memory T lymphocytes largely unknown. This is partly due to a lack of appropriate in vivo animal models. For example, mice lacking Bcl-xL and Mcl-1 die embryonically. Mice lacking Bcl-2 die within 3 weeks of birth, precluding the use of these animals to examine T cell immune response in the context of pathogenic infections. We have generated mice conditionally lacking Bcl-x, Bcl-2, and Mcl-1 in T lymphocytes. In addition, we have also generated mice in which Bcl-2 expression is genetically marked. These animal models enable us to address the roles of these anti-apoptotic molecules in regulating the survival of naive, effector, and memory T lymphocytes in vivo using Listeria monocytogenes infection model. Our overall hypothesis is that Bcl-2 and Mcl-1 differentially regulate T cell survival. We propose that on one hand, Bcl-2 primarily promotes the survival of memory T cells, while Mcl-1 is required for the survival of activated/effector T cells. On the other hand, we propose that both Bcl-2 and Mcl-1 promote naive T cell survival, but through distinct mechanisms. To test the above hypothesis, we propose three specific aims: 1: To examine the role of Bcl-2 and Mcl-1 in memory T lymphocyte development. 2: To elucidate the mechanisms by which Mcl-1 protects activated T cells from death. 3: To establish the mechanisms by which Bcl-2 and Mcl-1 protect naive T cells from death. Results from our proposed research will not only establish the roles of these important anti-apoptotic proteins in T cell survival, but also provide novel insights into boosting effector and memory T cell response to microbial pathogens by enhancing their survival.